Satellites and spacecrafts miniaturization is a recognized trend covering the whole range of space missions from simple university missions designed by students to sophisticated military satellites and interplanetary probes. The rationale for miniaturization comes from the drastic launch and manufacturing cost reductions in combination with the growing offer and availability of new miniaturized parts, subsystems and satellite buses.
Besides such scientific missions, the first obvious tasks for micro and nano-satellites are Earth observation for environmental, military and geological uses, while telecommunications will follow as soon as the other technological building blocks will be available. Last but not least, missions related to entertainment and space tourism will become common as soon as rocket plane launches will be offered on regular basis.
In fact, the so called “New Space” movement will be finally able to solve the major obstacle to the diffusion of small satellites: the launch. Until now, launches are always arranged in piggyback fashion whereby the largest cost of the launch is paid by a very big satellite and the nano or microsatellite are taken in an orbit and with a schedule which are not necessarily the optimal for their needs and interests. The space tourism technologies developed by the “New Space” entrepreneurs will enable weekly or daily regular launches from Spaceports distributed all over the world. In this way the full potential of micro and nano-satellites will be finally available possibly having extremely short time missions, launched on demand and with hardware of very low cost.
The usual definition of satellite sizes classifies Microsatellites between 10 kg and 100 kg, Nano-satellites between 1 kg and 10 kg and Pico-satellites below 1 kg.
Among the nano-satellites the Cubesat ranges from 1 kg to 3 kg with a nominal cross section of 100 mm×100 mm and nominal length between 50 mm and 350 mm. This standard, introduced by Stanford University and California Polytechnic is extremely important because of the use of a standardized deployment system called P-Pod (5) which completely de-couples the integration of the nano-satellites from the rocket vehicle making very simple the launching of a nano-satellite even for a small university team. All rocket interface issues are taken care by the P-Pod standard deployment system. Various commercial ventures are even promoting satellites of 750 g, of cylindrical shape, called “Tubesats”, for US$ 8000 launch included on very Low Earth Orbit (LEO) with the first launch scheduled for 2011.
One of the improvements long waited for will be the advent of really miniaturized and efficient propulsion systems which in combination with other subsystems miniaturization efforts will allow the use of micro and nano-satellites for comparable to the more conventional and larger spacecrafts ones.
A challenge for achieving a small thrust is to do it efficiently in order to have the best use of the limited amount of propellant available on board This as well as thrusters efficiencies have to cope with the mission requirements of spacecraft velocity change ΔV.
Generally speaking a list of typical manoeuvres with an indication of ΔV ranges from is:                Transfer to planetary trajectory (3600 to 4000 m/s)        Orbit transfer to GEO (−4000 m/s)        Plane change (100-1000 m/s)        Orbit rising, drag make-up, controlled re-entry (50˜1500 m/s)        Orbit maintenance and attitude control (10-100 m/s per year)        Relative motion of spacecrafts (1-100 m/s per manoeuvre)        
To this end, for a nano-satellite, the standard design performance expected may fall within the following parameters:                Thrust between 500 μN and 500 mN;        Minimum ΔV 10 m/s;        Average ΔV 100 m/s (includes orbit injection correction, acquisition and maintenance), up to 200 m/s if including de-orbiting;        Maximum ΔV 2000 m/s (including Moon, transfers or orbit changes);        Dry mass budget 1 kg (possibly 400 g);        Propellant mass budget 3 kg;        Tank size max 200 mm diameter;        Power budget, max 10 W, average 2˜3 W;        1 DoF for high ΔV, 3 DoF for low AA, 6 DoF for moderate ΔV, μN for attitude control.        
Since low thrust propulsion systems have become available few decades ago, we have learned how to benefit from continuous-thrust manoeuvres instead of impulsive manoeuvres. This is going to be even more important for nano and microsatellites which necessarily have extremely low power availability and small mass budget for propellant storage, requiring high specific impulse engines working at low thrust for long time.
The selection of a propulsion technology for a given spacecraft and mission requires consideration of the whole system whereby the engine is accompanied by a reservoir or tank and a power supply which includes batteries and solar cells.
Propulsion systems include the five main groups of elements: mass storage and supply, electric storage and supply, thermodynamic acceleration of the propellant, propellant ionization, propellant electrical acceleration. The various combination of such elements create the different propulsion system which all have the common goal to produce the highest possible momentum of the ejected propellant with the smallest possible use of propellant mass and electric power.
Chemical propulsion systems derive their energy from the chemical energy content of the propellant which is endo-thermically heated or, in addition to it electro-thermally heated, leading to the achievement of propellant exit velocity which depends on the achievable propellant temperature (24).
  =                                          2            ⁢                                                  ⁢            γ                                γ            -            1                              ⁢      Γ      ⁢                          ⁢                                    C            *                    ⁡                      [                          1              -                                                (                                                            P                      s                                                              P                      c                                                        )                                                                      γ                    -                    1                                    γ                                                      ]                                    1          2                      ⁢                  =                            (                                    γ              +              1                        2                    )                                      (                          γ              +              1                        )                    /                      (                          γ              -              1                        )                              ⁢                                    RT            t            *                    γ                    
The temperature achievable by the propellant is ultimately limited by the combustion chamber and expansion nozzles materials giving a practical limitation to the specific impulse of chemical propulsion systems.
Propulsion systems will soon enable very advanced small satellite missions including constellation and formation flight with distributed sensors, communication networking, assembly of larger structures in Space, maintenance of larger spacecrafts, de-orbiting, Moon exploration and others. While many laboratories worldwide are studying and prototyping systems based on various principles, there are very few examples of micro and nano-satellites carrying a. micro-propulsion unit. Furthermore, such units are generally limited to a micro-propulsion payload and do not perform a primary mission requirement. This is clearly due to the modest performances of the systems available to create a new micro-propulsion system really miniaturized in all its components providing sufficient Thrust, Specific Impulse and Efficient use of the limited available power and storage volume and mass budget.